The inner liner is disposed inwardly in a tire, and has a function of reducing an amount of leakage of air from inside to outside of the pneumatic tire to maintain tire internal pressure constant. As a material having such a function, a rubber composition having low air permeability, such as a butyl rubber, is conventionally used. On the other hand, in order to achieve weight reduction of a tire, a film made of a material containing a thermoplastic resin is used in some cases, instead of the above-described rubber composition.
Here, during use of the tire, large shear strain acts on a vicinity of a shoulder portion in the inner liner. When the material including the thermoplastic resin is used as the inner liner, this shear strain is likely to cause detachment at an adhesion interface between the inner liner and the carcass ply, with the result that air leakage takes place from the tire, disadvantageously.
On the other hand, low fuel consumption is demanded of a pneumatic tire, and reduction in rolling resistance by means of weight reduction of the tire is an issue. Therefore, a technique also has been proposed to employ a thermoplastic elastomer for the inner liner. However, when the inner liner of an elastomer is made thinner in thickness than that of a butyl rubber, it is difficult to achieve improved air permeability resistance and reduced weight at the same time. Moreover, reduction in thickness disadvantageously results in reduction in strength of the inner liner and damage or deformation of the inner liner due to heat and pressure of a bladder during a vulcanization step.
PTD 1 (Japanese Patent Laying-Open No. 9-19987) discloses a layer stack for improving adhesive property of an inner liner layer and a rubber layer. With this technique, adhesive layers are provided on both the sides of the inner liner layer. The adhesive layers come into contact with each other at an overlapping portion of the inner liner layer, and are adhered firmly to each other by heating, which improves air pressure retention property. However, these adhesive layers for overlapping of the inner liner layer will be brought into contact with the bladder in a heated state in a vulcanization step, and will be adhered to the bladder disadvantageously.
In PTD 2 (Japanese Patent No. 2999188), a blend of a nylon resin having favorable air permeability resistance and a butyl rubber is prepared by dynamic cross-linking, and a 100-μm-thick inner liner layer is produced. However, nylon resin is hard at a room temperature and is unsuitable as an inner liner for tire. Moreover, since this blend prepared by dynamic cross-linking will not be adhered by itself to a rubber layer through vulcanization, an adhesive layer for vulcanization is required in addition to the inner liner layer, resulting in an inner liner component of complicated structure and increase in the number of steps, which is disadvantageous from a viewpoint of productivity.
In PTD 3 (Japanese Patent Laying-Open No. 2008-24219), a maleic anhydride modified hydrogenated styrene-ethylene-butadiene-styrene block copolymer is dispersed in an ethylene-vinyl alcohol copolymer having favorable air permeability resistance to produce a flexible gas barrier layer. Furthermore, a sandwich structure is prepared by a thermoplastic polyurethane layer, and further, a rubber cement is applied to a surface to be adhered to a tire rubber (butyl rubber/natural rubber 70/30 is dissolved in toluene) to produce an inner liner layer.
However, the flexible resin-dispersed modified ethylene-vinyl alcohol copolymer has low adhesive strength, and is likely to be detached from the thermoplastic polyurethane layer. Moreover, although the flexible resin-dispersed modified ethylene-vinyl alcohol copolymer has flexible resin dispersed therein, EVOH of a matrix has poor flex fatigue resistance, and will be damaged during traveling with the tire. Furthermore, although the rubber cement is applied to the surface to be adhered to the tire rubber, an additional step will be necessary besides the usual inner liner step, which will result in poor productivity.
PTD 4 (Japanese Patent Laying-Open No. 2005-343379) achieves improved low temperature durability by designing the thickness at a shoulder portion to be larger than the thickness at a tire crown portion. However, increase in thickness dimension will result in increased weight, which is not preferable from a viewpoint of low fuel consumption and low manufacturing cost.